magnetism and electromagnetism

• Electromagnets are made by wrapping a coil of wire around a ferromagnetic core and passing an electric current through the wire.
• A compass needle is attracted to the north end of a bar magnet, indicating that it has a south pole at its north end.
• Two magnets will either attract or repel each other in the following way:

  • like poles (N-N or S-S) repel
  • unlike poles (N-S or S-N) attract

  Magnetic forces are non-contact forces - this means that magnets affect each other without touching.
• Iron, steel, nickel and cobalt are magnetic materials.
• A permanent magnet is often made from a magnetic material such as iron. A permanent magnet always causes a force on other magnets, or on magnetic materials. Key features of a permanent magnet:

  • it produces its own magnetic field
  • the magnetic field cannot be turned on and off - it is there all the time
• an induced magnet only becomes a magnet when it is placed in a magnetic field. The induced magnetism is quickly lost when the magnet is removed from the magnetic field.

  Like all induced magnets:

  • they are only attracted by other magnets, they are not repelled
  • they lose most or all of their magnetism when they are removed from the magnetic field
• A permanent magnet can:

  • attract or repel another permanent magnet
  • attract a magnetic material (but not repel it)

  This means that you can only show that an object is a permanent magnet by checking if it repels another magnet.
• A magnetic field is invisible but can be detected using a magnetic compass
• A compass contains a small bar magnet on a pivot that can rotate and points in the direction of the Earth's magnetic field or the magnetic field of a magnet
• Magnetic fields can be mapped out using small plotting compasses by:

  • Placing the plotting compass near the magnet on a piece of paper
  • Marking the direction the compass needle points
  • Moving the plotting compass to many different positions in the magnetic field, marking the needle direction each time
  • Joining the points to show the field lines
• The needle of a plotting compass points to the south pole of the magnet
• The behaviour of a compass indicates that the Earth has a magnetic field produced by the Earth's core, made from iron and nickel
• The diagram shows the magnetic field around a bar magnet
• A solenoid consists of a wire coiled up into a spiral shape. When an electric current flows, the shape of the magnetic field is very similar to the field of a bar magnet. The field inside a solenoid is strong and uniform. The small magnetic fields caused by the current in each coil add together to make a stronger overall magnetic field.

  
• When a current flows in a wire, it creates a circular magnetic field around the wire. This magnetic field can deflect the needle of a magnetic compass. The strength of the magnetic field is greater:

  • closer to the wire
  • if the current is increased

  
• A solenoid with an iron core is called an electromagnet. The iron core increases the solenoid’s magnetic field strength. A simple electromagnet is made by coiling wire around an iron nail.
• A wire carrying a current creates a magnetic field. This can interact with another magnetic field, causing a force that pushes the wire at right angles. This is called the motor effect.

  
• Calculating the motor effect force

  To calculate the force on a wire carrying a current at right angles to a magnetic field, use the equation:

  force = magnetic flux density × current × length

  2 A flows through a 50 cm wire. Calculate the force acting on the wire when it is placed at right angles in a 0.4 T magnetic field.

  50 cm = 50 ÷ 100 = 0.5 m

  F=B I L

  F=0.4×2×0.5, force = 0.4 N
• There is no motor effect force if the current and magnetic field are parallel to each other. The force on a given length of wire in a magnetic field increases when:

  • the current in the wire increases
  • the strength of the magnetic field increases

  
• A coil of wire carrying a current in a magnetic field experiences a force that tends to make it rotate. This effect can be used to make an electric motor.
• Current in the left-hand part of the coil causes a downward force, and current in the right-hand part of the coil causes an upward force
• The coil rotates anticlockwise due to the forces described above
• When the coil is vertical, it moves parallel to the magnetic field, producing no force
• Two features allow the coil to continue rotating:

  • The momentum of the motor carries it on round a little
  • A split ring commutator changes the current direction every half turn
• Once the conducting brushes reconnect with the commutator after a half turn:

  • Current flows in the opposite direction through the wire in the coil
  • Each side of the coil is now near the opposite magnetic pole
• The motor effect forces continue to cause anticlockwise rotation of the coil
• The motor effect is used inside headphones, which contain small loudspeakers. In these devices, variations in an electric current cause variations in the magnetic field produced by an electromagnet. This causes a cone to move, which creates pressure variations in the air and forms sound waves. To make a loudspeaker cone vibrate correctly, the electric current must vary in the same way as the desired sound.

  
• Alternating current supplied to the loudspeaker creates sound waves in the following way:

  1. a current in the coil creates an electromagnetic field
  2. the electromagnetic field interacts with the permanent magnet generating a force, which pushes the cone outwards
  3. the current is made to flow in the opposite direction
  4. the direction of the electromagnetic field reverses
  5. the force on the cone now pulls it back in
  6. repeatedly alternating the current direction makes the cone vibrate in and out
  7. the cone vibrations cause pressure variations in the air, which are sound waves
• An alternating current (ac) generator is a device that produces a potential difference. A simple ac generator consists of a coil of wire rotating in a magnetic field. Cars use a type of ac generator, called an alternator to keep the battery charged and to run the electrical system while the engine is working.

  
• As one side of the coil moves up through the magnetic field, a potential difference is induced in one direction. As the rotation continues and that side of the coil moves down, the induced potential difference reverses direction. This means that the alternator produces a current that is constantly changing. This is alternating current or ac.
• The graph shows an alternating sine curve. The maximum potential difference or current can be increased by:

  • increasing the rate of rotation
  • increasing the strength of the magnetic field
  • increasing the number of turns on the coil

  
• A direct current (dc) generator is another device that produces a potential difference. A simple dc generator consists of a coil of wire rotating in a magnetic field. However, it uses a split ring commutator rather than the two slip rings found in alternating current (ac) generators. Some bike lights use a type of dc generator called a dynamo to run the lamps while the wheels are turning.
• In a dynamo, a split ring commutator changes the coil connections every half turn. As the induced potential difference is about to change direction, the connections are reversed. This means that the current to the external circuit always flows in the same direction.
• The maximum potential difference or current can be increased by:

  • increasing the rate of rotation
  • increasing the strength of the magnetic field
  • increasing the number of turns on the coil

  The diagram shows four different positions of the coil in a dynamo, and the corresponding potential difference produced.

  
• The microphone is a device that converts sound waves into electrical signals. Microphones use the generator effect to induce a changing current from the pressure variations of sound waves.

  
• 1. pressure variations in sound waves cause the flexible diaphragm to vibrate
  2. the vibrations of the diaphragm cause vibrations in the coil
  3. the coil moves relative to a permanent magnet, so a potential difference is induced in the coil
  4. the coil is part of a complete circuit, so the induced potential difference causes a current to flow around the circuit
  5. the changing size and direction of the induced current matches the vibrations of the coil
  6. the electrical signals generated match the pressure variations in the sound waves
• A basic transformer is made from two coils of wire, a primary coil from the alternating current (ac) input and a secondary coil leading to the ac output. The coils are not electrically connected. Instead, they are wound around an iron core. This is easily magnetised and can carry magnetic fields from the primary coil to the secondary coil.
• 1. a primary voltage drives an alternating current through the primary coil
  2. the primary coil current produces a magnetic field, which changes as the current changes
  3. the iron core increases the strength of the magnetic field
  4. the changing magnetic field induces a changing potential difference in the secondary coil
  5. the induced potential difference produces an alternating current in the external circuit